1. Field of the Invention
The present invention relates to a solder paste and a soldering method of the solder paste, more particularly, to a soldering paste which is used in a circuit plate for electronic components or the like and a soldering method of the solder paste.
2. Description of the Related Art
In recently years, with development of compact size of an electronic appliance, a surface mounting technology has been becoming a main one for a mounting technology. This technology involves the steps of screen-printing a solder paste on a printed board, mounting a component and then reflowing the solder paste in a reflow furnace to form a circuit board.
The solder paste is a key technical element for the surface mounting technology. In order to solder the solder paste to a fine terminal with high reliability, a need exists for a solder paste which has high performance and high reliability.
It is well-known that when a junction metal (for example, an electrode of the electronic component) is soldered to another junction metal to be bonded (for example, an electrode of the printed board), it is necessary for the solder paste to have an solder alloy and solder flux. This solder flux can serve as (1) cleaning a surface of metal participating in a junction (removal of an oxide film), (2) re-oxidation preventing the cleaned metal surface, and (3) decreasing a surface tension of a fused solder.
The solder paste comprises a solder alloy (a solder powder) which is powdered to have a particle size of a few ten xcexcm, and an organic component (that is to say, flux vehicle) which performs functions of providing a role as the flux and thixotropic properties to the solder paste. The performance to be required for the above solder paste involves solderability (wettability), printability and stability.
Properties of the solder powder and the flux vehicle play an important role on these abilities. For instance, the printability is highly dependent on the shape, the particle size and the particle size distribution of the solder powder, components of the flux, such as a solvent, a thixotropic agent and rosin of the flux vehicle, and mixing and tempering conditions for the solder powder and flux vehicle in making the solder paste.
Among the performances of the solder paste, since solderability has a significant influence on reliability of a connection portion between the circuit board and the components, this solderability dictates reliability of products. This solderability is strongly dependent upon a surface state and the particle size of the solder powder, flux vehicle components, in particular, rosin and a type and a content of an activating agent, or the like. Accordingly, up to the present, in order to improve solderability of the solder paste, it is normal to adjust the surface state and the particle size of the solder powder, flux vehicle components, in particular, rosin and the type and the content of the activating agent, or the like.
The activating agent acts directly on an oxide on a surface of the metal when heating is carried out during soldering operation, this agent has the function as the flux, this function of the activating agent being more excellent than that of only rosin. The activating agent is known to be hydrogen halide acid salts of amines, and organic acids. As the hydrogen halide acid salts of amine, in general, use is made of amine-HCl and amine-HBr. Activity of the hydrogen halide acid salts of amines is dependent on a molecular structure of amine.
However, in general, it is well-known that a rosin activated highly showing a high activity comprises amine-HCl and amine-HBr can be used as a slight amonut of the activating agent for a rosin activated midly. There is a good relationship between reliability and solubility of halide residues derived from hydrogen halide acid salts of amine against water. If these residues are not dissolved in water, there is no risk of corrosion.
On the other hand, the organic acids are typically used in a combination with a halogen salt of amine. The organic acids have the advantages that most organic acids exhibit an activating power from a low temperature at which an amount of the added halide can be inhibited, which results in low reliability of the products. The activating power of these organic acids is determined by acidity, that is, dissociation constant of acid. In general, the lower molecule has better acidity and shows the good activating power as the flux.
However, although the lower molecule has the large activating power as explained above, this molecule has a low melting point and it tends to volatilize upon making repeated use of the paste or upon allowing to stand for a waiting period from a screen-printing step to a soldering step. This may result in deterioration of soldering.
In addition, since the organic acid of the lower molecule is very reactive at room temperature, this organic acid which is contains in the solder paste may induce corrosion of the solder powder and may result in low shelf life of the solder paste. Moreover, because the organic acid of the reactive lower molecule has an offensive smell, there may be arisen problems that it is difficult to handle the solder paste containing the above compound.
By the way, recently, much attention has been paid to environmental problems and there has been a trend toward regulations against lead contained in the solder which is used for the electronic appliances. That is to say that development of xe2x80x9clead-free solderxe2x80x9d have been made extensively.
As a development goal of the lead-free solder, this goal is to have the same properties as a conventional Snxe2x80x94Pb eutectic solder which is a composite material and to have no load towards the environment. As a good candidate for the lead-free solder, there are Snxe2x80x94Ag eutectic, Snxe2x80x94Bi eutectic, and Snxe2x80x94Zn eutectic systems or the like. There is developed a new alloy in which additional third and forth elements are added into each eutectic composition for the purpose of improving the properties, such as mechanical property and the melting point. However, any of alloys developed up to the present have some advantages and other disadvantages as the solder material, a solder paste having a high reliability has not been developed in the present time.
Speaking of solderability of this lead-free solder (concerning wettability, spreadability and capability of generating a solder ball), solderability of most Snxe2x80x94Bi eutectic and Snxe2x80x94Zn eutectic systems is inferior to that of Snxe2x80x94Pb eutectic system, so that many problems are not settled as far as the former systems are concerned.
In order for the above Snxe2x80x94Bi eutectic system to have effective solderability at low temperature, it is usual to add the activating agent which exhibits the activating power at low temperature into the solder paste. As the activating agent which has the activating power at low temperature, there may be used many low molecular organic acids having a carboxylic acid. However, when these low molecular organic acids are used as the activating agent, as stated above, there may be arisen many problems relating to deterioration of solderability due to volatilization of the organic acid and corrosion of the solder powder caused by high reactivity of the organic acid, low shelf life induced by the foregoing, low operational effectiveness by the bad odor of the organic acid.
Accordingly, it is a general object of the present invention is to provide a solder paste with which properties required for soldering can be improved.
A more specific object of the present invention is to provide a solder paste with which solderability, corrosive resistance and a shelf life can be ameliorated, and low operational effectiveness due to a bad odor of an organic acid can be improved.
The above objects of the present invention are achieved by a solder paste comprising: a solder powder; and a flux comprising at least an activating agent, and acid anhydride obtainable by a dehydration reaction of aliphatic carboxylic acid having up to seven carbon atoms.
The above-mentioned objects of the present invention can also be achieved by a solder paste comprising: a solder powder; and a flux containing at least an activating agent and acid anhydride having the following formula as shown below:
(CH3(CH2)nCO)2O
where n is a integer from 0 to 2.
Another object of the present invention is to provide a soldering method by use of a solder paste comprising: a solder powder; and a flux comprising at least an activating agent, and acid anhydride obtainable by a dehydration reaction of aliphatic carboxylic acid having up to seven carbon atoms.
The above object of the present invention is achieved by a soldering method comprising the step of: mixing a solder powder with a flux comprising at least an activating agent and acid anhydride obtainable by a dehydration reaction of aliphatic carboxylic acid having up to seven carbon atoms so as to form a solder paste; placing said solder paste in a predetermined position; and soldering a solder alloy with said solder paste, said solder alloy having a lower melting point thereof than that of Snxe2x80x94Pb eutectic solder.
According to the present invention, there is provided a soldering method comprising the step of: mixing a solder powder with a flux comprising at least an activating agent and acid anhydride obtainable by a dehydration reaction of aliphatic carboxylic acid having up to seven carbon atoms so as to form a solder paste; placing said solder paste in a predetermined position; and soldering a solder alloy with said solder paste, said solder alloy comprising from 40 to 60 percent by weight of Sn and from 60 to 40 percent by weight of Bi.
An advantage of the present invention is the provision of a solder paste having excellent solderability, high corrosive resistance and high shelf life.